For thermoplastic polyester resins, the fibrous glass enhances the mechanical properties of the resin. Glass filled crystalline resin blends are difficult to mold due to a combination of poor mold flow and shear sensitivity. Lack of these properties make thin wall molding such as may be desired with blow molding, injection molding and extrusion molding techniques difficult to achieve. The crystalline resin has poor melt strength and high shrinkage upon cooling. This makes it difficult to obtain good dimensional tolerances. Typically, extruded blends also have a very rough surface.
Unfortunately, in such resins, the addition of glass fibers can substantially lower the Izod impact strengths of the fiber reinforced compositions, and, also, substantially reduce the biaxial impact (instrument impact) energies of such compositions.
Holub et al, U.S. Pat. No. 4,122,061 describes polyester reinforced blends of poly(1,4-butylene terephthalate) and poly(ethylene terephthalate) impact modified with the addition of a polyolefin or olefin based copolymer resin. The polyolefinic resin is dispersed into the polyester blend and used for injection molding. Workpieces molded from such polyester resins are described as having a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction. It is desirable to provide further enhancements to the properties of resins of the type described in Holub et al.
U.S. Pat. No. 5,441,997 describes polyester molding compositions which have ceramic like qualities, can be molded into relatively thin sections, and have high impact strength. The composition is directed to a polybutylene terephthalate and/or polyethylene terephthalate and an aromatic polycarbonate with inorganic fillers selected from the group consisting of barium sulfate, strontium sulfate, zirconium oxide and zinc sulfate. If desired, a styrene rubber impact modifier is described as an added to the composition as well as a fibrous glass reinforcing filler. Although these Ecompositions are suited for applications where ceramic like qualities are desired, compositions for profile type extrusions are having different property balance is desirable.
The prior art focus is to add various ingredients to enhance the properties of the polyester material. However, with this approach, it is difficult to achieve the desired improvement in properties. The addition of one ingredient may improve one property but result in the degradation of another desirable property. As a result, a combination of various ingredients are added which further complicates the production of a consistent product due to the interactions of the various ingredients with one another.
As an example of an additive used to improve properties, it is known to add impact modifiers to polyester resins to improve the ductility or the notched Izod impact strength thereof. However the effect of such impact modifiers on reinforcing polyester resin compositions, such as those containing 30% by weight of reinforcing glass fibers, is a relatively modest improvement in ductility and, unfortunately, an increase in the melt viscosity or reduction in the melt flow, modulus and heat distortion properties of the composition.
One problem with polyesters in injection molding applications is that it is sometimes difficult to fill large molds requiring long flow lengths or very thin parts because of the high melt viscosity of the polymer. A solution to this problem is to raise the temperature of the polymer melt, thus lowering its viscosity. This solution is often not satisfactory for polyester polymers since polyesters tend to degrade, or lose molecular weight, at elevated processing temperatures.
Also, there are other well known additives which reduce the melt viscosity of polyesters. However, these additives usually make the resulting molded material less rigid, less heat resistant, and/or function only by breaking down the molecular weight of the polyester resulting in reduced impact.
Accordingly, there is a need for a more simple glass reinforced polyester resin compositions which exhibit consistent and uniform properties together with enhanced properties of improve mold flow, increase shear sensitivity, increase tensile strength, and increase flexual modulus in glass filled material.